Advanced Warp ScreenSaver
Advanced Warp makes your desktop look like it's undergoing some strange transformation--melting, suddenly blurring and receding, or being covered by a storm front. You can set the screen saver to respond to mouse movement, so in the Shadow Storm mode you can make a cloud follow your cursor, and in one of the melting modes the distortion will follow your cursor.
The program is simple to install; just click the executable. The trial version has a large bright green nag screen that bounces around the desktop. Registering Advanced Warp for $15 removes the nag screen and gets you two additional effects, technical support, and a free upgrade to future versions.